New research has revealed that lab mice react differently to men and women — and this can influence test results.

Previous behavioural study results on lab mice may be in question after a factor was found to induce a stress response in the animals: namely, the presence of male experimenters in the lab. According to a study conducted by pain researchers and led by McGill University, the scent of men induces a stress response in mice equivalent to that caused by restraining the animal for 15 minutes or making it swim for three.

This stress response — used in preclinical trials to gauge responses to pharmaceuticals, for example — manifests in several ways, perhaps the most impactful of which is that it makes mice and rats less sensitive to pain — up to about 35 per cent less.

"Scientists whisper to each other at conferences that their rodent research subjects appear to be aware of their presence, and that this might affect the results of experiments, but this has never been directly demonstrated until now," said senior author Professor Jeffrey Mogil.

The difference between men and women that elicited a stress response in the mice was smell, the researchers found. They tested this by having both men and women wear T-shirts, which were placed next to the mice the following day. This produced exactly the same result. This was also observed when bedding used by male animals was placed near the mice — although the effect was not replicated, interestingly, by the bedding of desexed male dogs.

The culprit was found to be chemosignals, or pheromones, secreted in the armpit at higher levels in men than in women. All mammals secrete the same chemosignals, and rodents rely on them to detect the presence of nearby male animals.

The team also checked previous research based on archival data, and found that the results corresponded with the new research.

Now that the information is known, however, scientists going forward will be able to take steps to control for it. "The problem is easily solved by simple changes to experimental procedures," Professor Mogil said. "For example, since the effect of males' presence diminishes over time, the male experimenter can stay in the room with the animals before starting testing. At the very least, published papers should state the gender of the experimenter who performed the behavioral testing."